1. Field of the Invention
The present invention relates to a voltage regulator, and more particularly, to a voltage regulator having an overcurrent protection function.
2. Description of the Related Art
FIG. 4 is a circuit diagram for illustrating a related-art voltage regulator 300.
The related-art voltage regulator 300 includes a power supply terminal 301, a ground terminal 302, a reference voltage source 310, an error amplifier circuit 311, resistors 312, 317, 318, and 319, an NMOS transistor 316, PMOS transistors 313, 314, and 315, and an output terminal 320.
The PMOS transistor 315 has a source connected to the power supply terminal 301, and a drain connected to the output terminal 320 and one end of the resistor 318. The resistor 318 has another end connected to one end of the resistor 319 and a non-inverting input terminal of the error amplifier circuit 311. The resistor 319 has another end connected to the ground terminal 302. The PMOS transistor 314 has a source connected to the power supply terminal 301, and a drain connected to one end of the resistor 317 and a gate of the NMOS transistor 316. The PMOS transistor 313 has a source connected to the power supply terminal 301, a drain connected to a gate of the PMOS transistor 315, a gate of the PMOS transistor 314, and an output of the error amplifier circuit 311. The resistor 312 has one end connected to the power supply terminal 301, and another end connected to a gate of the PMOS transistor 313 and a drain of the NMOS transistor 316. The error amplifier circuit 311 has an inverting input terminal connected to one end of the reference voltage source 310. The reference voltage source 310 has another end connected to the ground terminal 302. The NMOS transistor 316 has a source connected to the ground terminal 302.
The related-art voltage regulator 300 operates such that, through a negative feedback circuit forming of the error amplifier circuit 311, the PMOS transistor 315, and the resistors 318 and 319, a voltage at the one end of the resistor 319 is equal to a voltage VREF at the reference voltage source 310.
When a current that flows to a load (not shown) connected to the output terminal 320 increases in this state, a drain current I1 of the PMOS transistor 315 increases. Then, a drain current I2 of the PMOS transistor 314, which is formed to have a predetermined size ratio to the PMOS transistor 315, also increases. The current I2 is supplied to the resistor 317 such that a voltage Vx is generated at the one end of the resistor 317. When the voltage Vx increases to exceed a threshold of the NMOS transistor 316, the NMOS transistor 316 is turned on, to thereby generate a drain current. The drain current of the NMOS transistor 316 is supplied to the resistor 312, such that a voltage at the other end thereof decreases, to thereby turn on the PMOS transistor 313. When the PMOS transistor 313 is turned on, a gate voltage of the PMOS transistor 315 increases, thereby limiting the drain current I1.
Now, when a resistance value of the resistor 317 is represented by R1, the size ratio between the PMOS transistors 315 and 314 is represented by K, and a threshold voltage of the NMOS transistor 316 is represented by |VTHN|, a limited current I1m of the current I1 is expressed by Expression (1).
                              I          ⁢                                          ⁢          1          ⁢          m                =                              K            ×            VTHN                                R            ⁢                                                  ⁢            1                                              (        1        )            
As described above, the related-art voltage regulator 300 has an overcurrent protection function, and an output current may be limited when the load is short-circuited, for example (see, for example, Japanese Patent Application Laid-open No. 2003-29856).
However, the related-art voltage regulator 300 has a problem in that fluctuation in the limited current I1m is large. This is because fluctuation in the threshold voltage VTHN affects the limited current I1m, as can be seen in Expression (1).
FIG. 5 is a graph for showing a waveform of an output voltage VOUT relative to an output current IOUT of the related-art voltage regulator 300. The dotted lines indicate a fluctuation range of the limited current. In general, the fluctuation in the threshold voltage VTHN is about ±0.1 from a center value of 0.6 V, and hence the fluctuation in the limited current I1m caused by the threshold voltage VTHN is ±16.7%, which is a very large fluctuation.